The idea of percentages of living among extinct species implied by the etymology of the first four terms has been so modified by modern discoveries that I have substituted for the more usual definitions those which are in harmony with our present knowledge, taking the most highly specialised animals as my guides, which alone have changed swiftly enough to be used to classify the subdivisions of the Tertiary period. The Prehistoric and Historic stages constitute the Recent division of Lyell and most other authors, and the evidence on which they, as well as the Pleistocene, are included in the Tertiary period will be placed before the reader in the course of this work.
The Specialisation of the Mammalia explained by the Theory of Evolution.
The argument in favour of the theory of evolution, founded on the specialisation of mammalian life, in its progress from the Eocene times down to the present day, seems to me so strong as to be almost irresistible. The facts are put into a tangible shape in the accompanying diagram (Fig. 1), in which it will be observed that the orders, families, genera, and species fall into the shape of a genealogical tree, with its trunk hidden in the Secondary period, and its branchlets, the living species, passing upwards from the Pleiocene, Pleistocene, and Prehistoric stages to the present time—a tree of life with living mammalia for its fruit and foliage. Were the extinct species taken into account, it would be seen that they fill in the intervals separating one living form from another, and that they grow more and more like the living forms as they approach nearer to the present day.
